The α-Chains of C4b-binding Protein Mediate Complex Formation with Low Density Lipoprotein Receptor-related Protein*

C4b-binding protein (C4BP) is a heparin-binding protein that participates in both the complement and hemostatic system. We investigated the interaction between C4BP and low density lipoprotein receptor-related protein (LRP), an endocytic receptor involved in the catabolism of various heparin-binding proteins. Both plasma-derived C4BP and recombinant C4BP consisting of only its α-chains (rC4BPα) bound efficiently to immobilized LRP, as determined by surface plasmon resonance analysis. Complementary, two distinct fragments of LRP, i.e. clusters II and IV, both associated to immobilized rC4BPα, and binding could be inhibited by the LRP antagonist receptor-associated protein. Further analysis showed that association of rC4BPα to LRP was inhibited by heparin or by anti-C4BP antibody RU-3B9, which recognizes the heparin-binding region of the C4BP α-chains. In cellular degradation experiments, LRP-expressing fibroblasts effectively degraded125I-labeled rC4BPα, whereas their LRP-deficient counterparts displayed a 4-fold diminished capacity of degrading125I-rC4BPα. Finally, initial clearance of C4BP in mice was significantly delayed upon co-injection with receptor-associated protein. In conclusion, our data demonstrate that the α-chains of C4BP comprise a binding site for LRP. We propose that LRP mediates at least in part the catabolism of C4BP and, as such, may regulate C4BP participation in complement and hemostatic processes.